Hetero-structured, inverted-T field effect transistor

ABSTRACT

The present invention provides a method of forming a transistor. The method includes forming a first layer of a first semiconductor material above an insulation layer. The first semiconductor material is selected to provide high mobility to a first carrier type. The method also includes forming a second layer of a second semiconductor material above the first layer of semiconductor material. The second semiconductor material is selected to provide high mobility to a second carrier type opposite the first carrier type. The method further includes forming a first masking layer adjacent the second layer and etching the second layer through the first masking layer to form at least one feature in the second layer. Each feature in the second layer forms an inverted-T shape with a portion of the second layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to semiconductor processing, and, moreparticularly, to forming a hetero-structured, inverted-T field effecttransistor.

2. Description of the Related Art

The constant drive to increase the density of semiconductor devices thatcan be formed on a wafer and the speed at which these devices operatehas led to many modifications in the structure of conventionalsemiconductor devices. For example, as development goals have approachedthe 22 nm node, attempts to maintain a conventional planar devicescaling have encountered roadblocks including inadequate control of thechannel region by the gate electrode, which may lead to short channeleffects. Multi-directional control of the channel may allow increasedimmunity to short channel effects such as sub threshold slopes, draininduced barrier leakage, and the like. Many semiconductor devices maytherefore be formed using multigate field effect transistors (FETs). Oneexample of a multi-gate FET incorporates a gate structure formed from anultrathin body (UTB) that is turned on end relative to conventionalplanar gate structures (i.e., the UTB gate structure is perpendicular tothe substrate). These devices are conventionally referred to as Fin-FETsbecause of the fin-like shape of the structures that connect the sourceand drain regions of the Fin-FETs to the gate structure. The Fin-FETdevices may offer a means of packing more current (and consequently morespeed) into each unit area of a chip while keeping the processing,materials, and circuit design factors relatively consistent withprevious technology nodes.

FIGS. 1A, 1B, and 1C conceptually illustrate a conventional method offorming fin structures in a Fin-FET. These figures depict across-sectional view 100 of the material layers used to form the finstructures. In the embodiment shown in FIG. 1A, a layer 105 ofsemiconductor material (e.g., monocrystalline silicon, silicongermanium, or germanium) is formed over a dielectric layer 110 (e.g.,silicon dioxide). An oxide layer 115 is then formed over the layer ofsemiconductor material 105. The oxide layer 115 may serve as aprotective layer for the layer 105 for subsequent forming and/or etchingof other structures. The oxide layer 115 may also serve as a stressreduction layer for subsequently deposited layers, such as nitridelayers. In some embodiments, another nitride layer 120 is deposited. Aphotoresist layer, which is patterned, using e.g. a mask, is then formedover the nitride layer and is used to transfer the pattern to thenitride layer. The patterned nitride layer 120 is used as a mask to etchthe oxide layer 115 and the layer 105 to form the fin structures 125shown in FIG. 1B. In one embodiment, the patterned nitride layer 120 andthe oxide layer 115 may be removed to leave behind the fin structures125 shown in FIG. 1C.

FIG. 2 conceptually illustrates a top-down view of a conventionaltransistor 200 formed using Fin-FET techniques. The transistor 200includes a gate electrode 205 that is positioned between a source 210and a drain 215. The fin structures 220, such as the thin structures 125shown in FIGS. 1A-C, have been formed between the source 210 and thedrain 215, and these structures extend underneath the gate electrode205. Examples of Fin-FETs and the techniques that may be used to formFin-FETs are found in Rao and Mathew (U.S. Pat. No. 7,265,059), Burnett,et al. (U.S. Patent Application Publication No. 2007/0161171), andHarris, et al. (“Fin-FETs: Challenges in Material and Processing for aNew 3-D Device Paradigm,” FUTURE FAB International, Issue 23).

The fin structures in conventional Fin-FETs may be configured to providerelatively high drive currents for the CMOS devices that incorporate theFin-FETs. However, the conventional fin structures have a singleorientation and are formed of a single material. Consequently,conventional fin structures can only be optimized to provide high drivecurrents for a single type of CMOS device, i.e. the fin structures canbe optimized for either a PMOS device where high hole mobility isdesired or an NMOS device where high electron mobility is desired. Mostcircuit designs include large numbers of both PMOS and NMOS devices. Theprocess flows used to form the circuit may be optimized for one type ofdevice, but this may also result in a less than optimal process flow forthe other type of device.

The subject matter described herein is directed to addressing theeffects of one or more of the problems set forth above.

SUMMARY

The following presents a simplified summary of the subject matterdescribed herein in order to provide a basic understanding of someaspects of the present invention. This summary is not an exhaustiveoverview of the present subject matter described herein. It is notintended to identify key or critical elements of the invention or todelineate the scope of the invention. Its sole purpose is to presentsome concepts in a simplified form as a prelude to the more detaileddescription that is discussed later.

In one embodiment, a method is provided for forming a transistor. Themethod includes forming a first layer of a first semiconductor materialabove an insulation layer. The first semiconductor material is selectedto provide high mobility to a first carrier type. The method alsoincludes forming a second layer of a second semiconductor material abovethe first layer of semiconductor material. The second semiconductormaterial is selected to provide high mobility to a second carrier typeopposite the first carrier type. The method further includes forming afirst masking layer adjacent the second layer and etching the secondlayer through the first masking layer to form at least one feature inthe second layer. Each feature in the second layer forms an inverted-Tshape with a portion of the second layer.

In another embodiment, a transistor is provided. The transistor includesa first layer formed of a first semiconductor material over a buriedoxide layer. The first semiconductor material is selected to providehigh mobility to a first carrier type. The transistor also includes asecond layer formed of a second semiconductor material adjacent thefirst layer. The second semiconductor material is selected to providehigh mobility to a second carrier type opposite the first carrier type.The second layer also includes at least one feature formed in the secondlayer by etching the second layer through a first masking layer suchthat each feature in the second layer forms an inverted-T shape with aportion of the first layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The present subject matter may be understood by reference to thefollowing description taken in conjunction with the accompanyingdrawings, in which like reference numerals identify like elements, andin which:

FIGS. 1A, 1B, and 1C conceptually illustrate a conventional method offorming fin structures in a Fin-FET;

FIG. 2 conceptually illustrates a top-down view of a conventionaltransistor formed using Fin-FET techniques;

FIGS. 3A, 3B, 3C, 3D, and 3E conceptually illustrate one exemplaryembodiment of a method of forming hetero-structured, inverted-T finstructures; as described herein;

FIG. 4 conceptually illustrates a top-down view of a transistorincluding hetero-structured, inverted-T fin structures as describedherein;

FIG. 5A conceptually illustrates a perspective view of a contiguousultra-thin body (UTB) device including hetero-structured, inverted-T finstructures; as described herein; and

FIG. 5B conceptually illustrates a perspective view of a multiple gateultra-thin body (UTB) device including hetero-structured, inverted-T finstructures; as described herein.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the scope ofthe invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Illustrative embodiments of the present subject matter are describedbelow. In the interest of clarity, not all features of an actualimplementation are described in this specification. It will of course beappreciated that in the development of any such actual embodiment,numerous implementation-specific decisions should be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

The present subject matter will now be described with reference to theattached figures. Various structures, systems and devices areschematically depicted in the drawings for purposes of explanation onlyand so as to not obscure the present invention with details that arewell known to those skilled in the art. Nevertheless, the attacheddrawings are included to describe and explain illustrative examples ofthe subject matter described herein. The words and phrases used hereinshould be understood and interpreted to have a meaning consistent withthe understanding of those words and phrases by those skilled in therelevant art. No special definition of a term or phrase, i.e., adefinition that is different from the ordinary and customary meaning asunderstood by those skilled in the art, is intended to be implied byconsistent usage of the term or phrase herein. To the extent that a termor phrase is intended to have a special meaning, i.e., a meaning otherthan that understood by skilled artisans, such a special definition willbe expressly set forth in the specification in a definitional mannerthat directly and unequivocally provides the special definition for theterm or phrase.

FIGS. 3A, 3B, 3C, 3D, and 3E conceptually illustrate one exemplaryembodiment of a method of forming hetero-structured, inverted-T finstructures. These figures depict a cross-sectional view 300 of thematerial layers used to form the hetero-structured, inverted-T finstructures. In the embodiment shown in FIG. 3A, a first layer 305 of asemiconductor material is formed adjacent a dielectric layer 310. Thesemiconductor material used to form the layer 305 may be selected toprovide relatively high hole or electron mobility. As used herein, theterm “adjacent” does not necessarily mean that the first layer 305 andthe dielectric layer 310 are in contact, although that may occur in someapplications. In some embodiments, one or more layers may be formedbetween the adjacent first layer 305 and dielectric layer 310. Thedielectric layer 310 may be a buried oxide layer formed of silicondioxide or other insulator and the first layer 305 may be a siliconlayer. Alternatively, the layer 305 may be a strainedsilicon-on-insulator layer 305. Techniques for forming silicon,silicon-on-insulator, and/or strained silicon-on-insulator layers 305are known to persons of ordinary skill in the art and in the interest ofclarity only those aspects of forming these layers 305 that are relevantto the present invention will be discussed herein.

Using the strained, tensile or compressive, silicon-on-insulator layer305 may allow mobility of electrons and/or holes in thehetero-structured, inverted-T fin structures to be controlled, modified,and/or increased. For example, straining the silicon-on-insulator layer305 may modify the conduction and/or valence band structure of thestrained silicon-on-insulator layer 305 to increase the mobility ofelectrons and/or holes. Furthermore, the strained silicon lattice in thestrained silicon-on-insulator layer 305 may also aid in growinghigh-quality epitaxial layers (e.g., the germanium layers discussedbelow) by reducing the possible lattice mismatch defects. Techniques forstraining silicon-on-insulator layer 305 to control mobility and/orreduce lattice mismatch defects, as well as to achieve other ends, areknown in the art and in the interest of clarity will not be discussedfurther herein.

As shown in FIG. 3B, a second layer 315 of a different type ofsemiconductor material is then formed above the first layer 305 so thatthe first layer 305 and the second layer 315 are adjacent each other.The semiconductor material used to form the second layer 315 may beselected to provide relatively high hole mobility, if the material usedfor first layer 305 is selected to provide relatively high electronmobility. For example, if the first layer 305 is formed ofsilicon-on-insulator then the second layer 315 may be formed ofgermanium. In one embodiment, the germanium layer 315 may beintrinsically doped and/or may be doped after deposition, e.g., usingion implantation techniques. Techniques for forming germanium layers 315are known to persons of ordinary skill in the art and in the interest ofclarity only those aspects of forming these layers 315 that are relevantto the present invention will be discussed herein. Alternatively, thesemiconductor material used to form the second layer 315 may be selectedto provide relatively high electron mobility, if the material used forfirst layer 305 is selected to provide relatively high hole mobility.

Although silicon and germanium are used to form the first and secondlayers 305, 315 in the illustrated embodiment, persons of ordinary skillin the art having benefit of the present disclosure should appreciatethat the present invention is not limited to forming the first andsecond layers 305, 315 from these materials. In alternative embodiments,other combinations of semiconductor materials may be selected so thatone of the layers 305, 315 provides relatively high electron mobilityand the other layer 305, 315 provides relatively high hole mobility.Exemplary combinations of materials that may be used to form the firstand second layers 305, 315 include using germanium to form the firstlayer 305 and silicon to form the second layer 315, using GaAs to formthe first layer 305 and germanium to form the second layer 315, andusing silicon to form the first layer 305 and GaAs to form the secondlayer 315.

As shown in FIG. 3C, an insulator layer 320, e.g., an oxide, is thenformed above the second layer 315. For example, the insulator layer 320may be formed by depositing an oxide (such as silicon dioxide) over thesecond layer 315. Alternatively, the insulator layer 320 may be formedby oxidizing an upper portion of the second layer 315. In oneembodiment, the insulator layer 320 may serve as a protective layerduring subsequent formation and/or etching of other structures. Anotherlayer 325 is then formed over the insulator layer 320, which can bepatterned using a photoresist. The patterned masking layer 325 may beformed of a variety of materials, e.g., a nitride layer, and the like.The pattern in the masking layer 325 reflects a desired pattern ofstructures that are to be formed by etching the second layer 315. Forexample, the pattern may define the dimensions (e.g., the width and thelength) of structures in the plane defined by the interface between thefirst and second layers 305, 315. The third dimension of the structures(e.g., the height) may be defined by the thickness of the second layer315.

The insulator layer 320 and the second layer 315 may then be etchedusing the patterned masking layer 325 as a mask, as shown in FIG. 3D.Etching of the second layer 315 may be stopped using etch stoptechniques (e.g., such as optical emission spectroscopy) or by timingthe duration of the etching process. Etching is preferably stoppedbefore the etching process begins to etch away portions of the firstlayer 305. However, persons of ordinary skill in the art having benefitof the present disclosure should appreciate that it may be difficult toprecisely control the etching so that none of the first layer 305 isetched. Thus, a realistic implementation of the etching process attemptsto stop the etching process when substantially all the unmasked portionsof the second layer 315 have been etched away (within some tolerance)and substantially none of the first layer 305 has been etched (withinsome tolerance). The etching process forms fin-shaped structures 330that include the un-etched portions of the insulator layer 320 and thesecond layer 315. In one embodiment, the width of the fin-shapedstructure 330 (i.e., the dimension in the plane of the paper) is smallerthan the depth of the fin-shaped structure 330 (i.e. the dimension thatis perpendicular to the plane of the paper). In one embodiment, thethickness of the fin-shaped structure 330 is approximately equal to thethickness of the insulator layer 320 and the second layer 315.

At this point in the processing, the fin-shaped structures 330 and thefirst layer 305 form a hetero-structured, inverted-T fin structure thatmay be used for a contiguous device such as a contiguous ultra-thin body(UTB) device. For example, the height of the fin-shaped structures 330may be substantially in the range of 15-90 nm and the thickness of thefirst layer 305 may be substantially in the range 1-60 nm. Theorientation of the semiconductor materials may be chosen to provide thehighest mobility of the associated carrier, e.g. holes or electrons. Inone embodiment, the first semiconductor material is chosen to have a(100) orientation and so the conduction planes 332 of the first layer305 have a planar (100) geometry and the conduction planes of the sidewall surfaces 334 of the second layer of the fin-shaped structures 330have a planar (110) geometry. Alternatively, a different orientation ofthe first semiconductor material may be chosen, e.g. a (110)orientation, to ensure higher carrier mobility, e.g. a higher holemobility. In this case the second semiconductor material would be formedwith another orientation, e.g. a (100) orientation.

In one alternative embodiment, a multiple gate UTB device may be formedby patterning the first layer 305. In the embodiment shown in FIG. 3E,portions of the first layer 305 are etched away to leave behind basestructures 335. Techniques for masking and/or etching portions of thefirst layer 305 are known in the art and in the interest of clarity willnot be discussed further herein. The base structures 335 are formedsubstantially symmetrically with the fin-shaped structures 330. Thewidth of the base structures 335 is larger than the width of thefin-shaped structures 330. For example, the width of a base structure335 may be approximately in the range of 60-200 nm and the width of thefin-shaped structures 330 may be approximately in the range 1-60 nm. Insome embodiment, the thickness of the base structures 335 may besubstantially the same as the thickness of the fin-shaped structures330. For example, the thickness of the fin-shaped structures 330 and thethickness of the base structures 335 may both be approximately 20 nm.However, this is merely intended as an illustrative example and may notapply in all cases. Consequently, the base structures 335 and thefin-shaped structures 330 form an inverted-T structure that extends(into the plane of the paper). In the illustrated embodiment exampledepicted herein, the thickness of the base structures 335 issubstantially equal to the thickness of the first layer 305.

FIG. 4 conceptually illustrates a top-down view of a transistor 400including hetero-structured, inverted-T fin structures 405. Two of thefin-structures 405 are approximately indicated by dashed ovals in FIG.4. In the illustrated embodiment, a gate 410, a source region 415, and adrain region 420 have been formed so that the fin-structures 405 willoperate as channel regions in the transistor 400. Techniques for formingthe gate 410, the source region 415, and the drain region 420, as wellas other elements of the transistor 400 such as the gate dielectric,contacts to the gate 410, the source 415, and/or the drain 420, andother backend processes are known in the art. In the interest ofclarity, only those aspects of forming the elements of the transistor400 that are relevant to the present invention will be discussed herein.

The fin-structures 405 shown in the illustrated embodiment include abase 425 that is formed of silicon and a fin 430 that is formed ofgermanium. In one embodiment, the fin structures 405 may be formedaccording to the process shown in FIGS. 3A-3E. The fin shaped structures405 are controlled by the gate 410 from multiple directions.Consequently, the transistor 400 may be more immune to short channeleffects than transistors that use conventional planar transistors. Thedevice geometry of the transistor 400 may be selected to optimize thematerial and/or orientation dependence of the hole and/or electronmobility to provide higher drive currents than transistors that use aconventional I-shaped (or bar shaped) fin, such as shown in FIG. 1C. Forexample, when the transistor 400 is implemented as a PMOS device, thegermanium portion, example portion 315, of the fin 430 providesrelatively high hole mobility. Furthermore, the (110) geometry of thesidewalls (not shown) of the fin 430 have a higher hole mobility thanthe (100) conduction planes in a planar geometry. The same transistor400 when implemented as an NMOS device, the (100) conduction plane inthe horizontal silicon-based 425 provides higher electron mobility.Symmetric electron mobility in the silicon base 425 and hole mobility inthe germanium fin 430 may also allow chip area optimization while stillproviding symmetric on-state currents in the transistor 400.

The transistor 400 may also be modified in other ways. For example, thechannel regions formed by the base 425 and the fin 430 may beintrinsically doped or can be doped, e.g., using ion implantation andannealing techniques. For another example, extensions may be formedusing extension implants or by using under lap of dopants from thesource region 410 and/or the drain region 415. For yet another example,parasitic resistances may be reduced by a selective epitaxial growth ofgermanium and/or silicon in regions outside of spacers formed in thetransistor 400.

FIG. 5A conceptually illustrates a perspective view of a contiguousultra-thin body (UTB) device 500. In the illustrated embodiment, the UTBdevice 500 is formed on a substrate 505 and includes fin-shapedstructures 510 and a first layer 515 that form a hetero-structured,inverted-T fin structure. The fin-shaped structures 510 are formed of afirst material that is selected to provide a relatively high mobilityfor a first carrier type, such as holes, and the first layer 515 isformed of a second material that is selected to provide a relativelyhigh mobility for a second carrier type, such as electrons. A layer 520is then formed over the hetero-structured, inverted-T fin structure. Thelayer 520 may be used to form gate, source and/or drain structures.

FIG. 5B conceptually illustrates a perspective view of a multiple gateultra-thin body (UTB) device 525. In the illustrated embodiment, the UTBdevice 525 is formed on a substrate 530 and includes fin-shapedstructures 535 and base structures 540 that form a hetero-structured,inverted-T fin structure. The fin-shaped structures 535 are formed of afirst material that is selected to provide a relatively high mobilityfor a first carrier type, such as holes, and the base structures 540 areformed of a second material that is selected to provide a relativelyhigh mobility for a second carrier type, such as electrons. A layer 545is then formed over the hetero-structured, inverted-T fin structure. Thelayer 545 may be used to form gate, source and/or drain structures.

The particular embodiments disclosed above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended to thedetails of construction or design herein shown, other than as describedin the claims below. It is therefore evident that the particularembodiments disclosed above may be altered or modified and all suchvariations are considered within the scope of the invention.Accordingly, the protection sought herein is as set forth in the claimsbelow.

1. A transistor, comprising: a first layer formed of a firstsemiconductor material over a buried oxide layer, the firstsemiconductor material selected to provide high mobility to a firstcarrier type; a second layer formed of a second semiconductor materialabutting the first layer, the second semiconductor material selected toprovide high mobility to a second carrier type opposite the firstcarrier type, and the second layer comprising at least one featureformed in the second layer such that said at least one feature in thesecond layer forms an inverted-T shape with a portion of the firstlayer, and wherein the portion of the first layer abuts said at leastone feature; a channel is formed in said at least one feature in thesecond layer and in the portion of the first layer, and a gate formedover the inverted structure.
 2. The transistor of claim 1, wherein thefirst layer is formed of a first semiconductor material selected toprovide high electron mobility, and wherein the second layer is formedof a second semiconductor material selected to provide high holemobility.
 3. The transistor of claim 1, wherein the first layer isformed of a first semiconductor material selected to provide high holemobility, and wherein the second layer is formed of a secondsemiconductor material selected to provide high electron mobility. 4.The transistor of claim 1, comprising a source and a drain connected bythe channel regions in said at least one feature in the second layer andthe portion of the first layer of the inverted-T structure.
 5. Thetransistor of claim 1, wherein the first layer comprises at least one ofa silicon layer, a silicon-on-insulator layer, a strainedsilicon-on-insulator layer, a germanium layer, and a GaAs layer.
 6. Thetransistor of claim 1, wherein the second layer comprises at least oneof a silicon layer, a silicon-on-insulator layer, a strainedsilicon-on-insulator layer, a germanium layer, and a GaAs layer.
 7. Thetransistor of claim 1, wherein said at least one feature formed in thesecond layer comprises a pattern of features having first and seconddimensions parallel to a plane of a surface between the first and secondlayers, the first dimension being smaller than the second dimension. 8.The transistor of claim 7, wherein said at least one feature formed inthe second layer has a third dimension perpendicular to the plane of thesurface between the first and second layers, the third dimension beinglarger than the first dimension.
 9. The transistor of claim 8,comprising at least one feature formed in the first layer and in contactwith said at least one feature formed in the second layer such that saidat least one feature in the first layer forms the base of an inverted-Tshape with a corresponding feature formed in the second layer.
 10. Thetransistor of claim 9, wherein said at least one feature formed in thefirst layer has planes of conduction that have a (100) geometry, andwherein said at least one feature formed in the second layer has planesof conduction that have a (110) geometry.
 11. The transistor of claim 9,wherein said at least one feature formed in the first layer has planesof conduction that have a (110) geometry, and wherein said at least onefeature formed in the second layer has planes of conduction that have a(100) geometry.
 12. A transistor, comprising: a first layer formed of afirst semiconductor material over a buried oxide layer, the firstsemiconductor material selected to provide a first mobility to a firstcarrier type; and a second layer formed of a second semiconductormaterial abutting the first layer, the second semiconductor materialselected to provide a second mobility to a second carrier type that isopposite to the first carrier type, and the second layer comprising atleast one feature formed in the second layer such that said at least onefeature in the second layer is in contact with a portion of the firstlayer to form an inverted-T shape with the portion of the first layerforming the base of the inverted-T shape; a channel formed in said atleast one feature in the second layer and the portion of the firstlayer, and a gate formed over the inverted-T structure.
 13. Thetransistor of claim 12, wherein the first layer is formed of a firstsemiconductor material that is strained to increase electron mobility,and wherein the second layer is formed of a second semiconductormaterial that is strained to increase hole mobility.
 14. The transistorof claim 12, wherein the first layer is formed of a first semiconductormaterial that is strained to increase hole mobility, and wherein thesecond layer is formed of a second semiconductor material that isstrained to increase electron mobility.
 15. The transistor of claim 12,comprising a source, a drain, and a gate that are connected byinverted-T structures formed of the first and second layers, and whereinthe inverted-T structures form channel regions when the gate isactivated.
 16. The transistor of claim 12, wherein a height of said atleast one feature in the second layer is substantially in the range of15-90 nm and a thickness of the first layer is substantially in therange 1-60 nm.
 17. The transistor of claim 12, wherein the portion ofthe first layer forming the base of the inverted-T shape has a widthapproximately in the range of 60-200 nm and a width of said at least onefeature is approximately in the range 1-60 nm.
 18. A transistor,comprising: an inverted-T structure comprising a first portion formed ofa first semiconductor material selected to provide high mobility to afirst carrier type and a second portion abutting the first portion,formed of a second semiconductor material, wherein the secondsemiconductor material is selected to provide high mobility to a secondcarrier type opposite the first carrier type; first and second channelregions formed in the first and second portions, respectively; and agate formed over the inverted-T structure.
 19. The transistor of claim18, wherein the first portion is formed of a first semiconductormaterial that is strained to increase electron mobility, and wherein thesecond portion is formed of a second semiconductor material that isstrained to increase hole mobility.
 20. The transistor of claim 18,wherein the first portion is formed of a first semiconductor materialthat is strained to increase hole mobility, and wherein the secondportion is formed of a second semiconductor material that is strained toincrease electron mobility.
 21. The transistor of claim 18, wherein thefirst portion has planes of conduction that have a (100) geometry, andwherein the second portion has planes of conduction that have a (110)geometry.
 22. The transistor of claim 18, wherein the first portion hasplanes of conduction that have a (110) geometry, and wherein the secondportion has planes of conduction that have a (100) geometry.